Selected Ion Flow Tube – Mass Spectrometry (SIFT-MS) study of the reactions of H3O+, NO+ and O2+ with a range of oxygenated volatile organic carbons (OVOCs)

Abstract

Selected Ion Flow Tube-Mass Spectrometry (SIFT-MS) uses soft chemical ionisation, typically by H3O+, NO+ and O2+, to analyse trace gases in real time at mixing ratios down to low parts per trillion by volume (pptv) and has recently been demonstrated as an effective method for monitoring volatile organic compounds (VOCs) in the atmosphere. SIFT-MS analysis requires accurate rate coefficient and product ion branching ratio data to allow for accurate mixing ratio calculations without the need for calibration standards. Most rate constant data have been measured using helium as the carrier gas. Here we report rate constants of a number of oxygenated volatile organic compounds (OVOCs) alongside benzene for H3O+, NO+ and O2+ reagent ions in nitrogen carrier gas, using a permeation tube calibration source and a certified gas standard to introduce known concentrations into the SIFT-MS. This includes the first reported literature rate constant for nonanal using SIFT-MS. We found that the rate constants for most compounds measured in nitrogen carrier gas were similar to those measured in helium. However, the NO+ rate constants for ketones measured in nitrogen were 1.2–5 times lower than those measured in helium. We observe higher fragmentation due to changes made to the lens voltages of the instrument in order to tune for higher sensitivity, altering the reagent ion energies. We also show the impact of increasing the flow tube temperature and voltage on the sensitivity and ionisation of benzene, butanone and butanal under both dry and humid conditions. Some of the SIFT-MS compound library entries are data from historic SIFT-MS instruments that lacked a heated flow tube, and therefore suffered from higher water clustering than current commercial SIFT-MS models. We show that a higher flow tube temperature reduces the amount of water adduct formation thereby simplifying compound concentration determination. These results demonstrate the need to ensure that the library data used are acquired under the same conditions as the experiment being performed.